Method and apparatus for measuring the charge-time response of materials by applying a voltage step across the same

ABSTRACT

A method and apparatus for measuring the charge-time response of an insulating material to a voltage step applied across it by applying to a capacitor containing a specimen of the material as dielectric, a voltage step or repeated voltage steps in the form of a pulse train, and sampling the charge on the specimen after a time interval from the application of the voltage step, said time intervals increasing logarithmically with successive samples. The charge-time response data obtained is particularly suited to the determination of permittivity and loss factor with frequency using approximations to the Fourier transform

United States Patent [191 Hyde eta]. [451 Feb. 6, 1973 [54] METHOD AND APPARATUS FOR [56] References Cited MEASURING THE CHARGE-TIME UNITED STATES PATENTS RESPONSE OF MATERIALS BY THE SAME Inventors: Peter John Hyde, Codicote, Hitchin,

- Hertfordshire; Wilson Reddish, London Colney, Hertfordshire, both of Appl. No.: 147,523

Related [1.8. Application Data Continuation-impart of Ser. No. 4,012, Jan. 19, 1970, abandoned.

US. Cl. ..324/61 R, 324/60 R Int. Cl. ..G0ln -27/26 Field of Search, .Q ..324/60, 61

Primary ExaminerAlfred E. Smith Attorney-Cushman, Darby & Cushman 57] ABSTRACT data obtained is particularly suited to the determination of permittivity and loss factor with frequency using approximations to the Fourier transform 24 Claims, 23 Drawing Figures PATENTEDFEB 6 I973 SHEET 01M 20 PATENTED E 6 I97 SHEET OEUF 2O PATENTEDFEB 61973 3.715.656

SHEET OBUF 20 l mm PATENTED E 197 3,715,856

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1. An apparatus for measuring the charge-time response of an insulating material to a voltage step applied across it, which apparatus comprises: a capacitor for containing a specimen of the material as dielectric, a step generator connected to said capacitor for applying thereto a voltage step or repetition of such voltage steps in the form of a pulse train, a detector arranged to detect the charge on the specimen and to provide a signal proportional To said charge at its output, a sampler connected to the output of the detector, and having gating means and logarithmic timing means controlling the operation of said gating means so as to obtain from the output of the detector after a time interval from the application of the voltage step or successive steps, successive samples of the signal representing the charge on the specimen in consequence of said voltage step or steps applied thereto, said time intervals being increased logarithmically with successive samples, and a data store arranged to receive the samples.
 1. An apparatus for measuring the charge-time response of an insulating material to a voltage step applied across it, which apparatus comprises: a capacitor for containing a specimen of the material as dielectric, a step generator connected to said capacitor for applying thereto a voltage step or repetition of such voltage steps in the form of a pulse train, a detector arranged to detect the charge on the specimen and to provide a signal proportional To said charge at its output, a sampler connected to the output of the detector, and having gating means and logarithmic timing means controlling the operation of said gating means so as to obtain from the output of the detector after a time interval from the application of the voltage step or successive steps, successive samples of the signal representing the charge on the specimen in consequence of said voltage step or steps applied thereto, said time intervals being increased logarithmically with successive samples, and a data store arranged to receive the samples.
 2. An apparatus according to claim 1, having a reference capacitor in which the dielectric has substantially no dielectric loss, said reference capacitor and the capacitor containing the specimen being connected in series by connecting means, said connecting means being also connected to the input of the detector; the step generator being a bipolar step generator arranged to apply equal voltage steps or pulses of opposite polarity substantially simultaneously to the two capacitors.
 3. An apparatus according to claim 1 in which the detector comprises an electrometer amplifier having an inputs stage comprising an insulated gate field effect transistor followed by one or more amplification stages, a capacitor being connected in parallel with said input and amplification stages to provide an output voltage proportional to the charge applied at the input stage.
 4. An apparatus according to claim 1 in which the sampler and data store are incorporated into a processing circuit comprising a sampling stage connected to the output of the detector, and having gating means for obtaining samples of the signal provided at the output of the detector and representing the charge on the specimen, a logarithmic clock connected to said gating means to provide control signals for the operation of the gating means in obtaining said samples at successive logarithmically separated times after the application of a voltage step to the specimen, a subtracting means connected to the sampling stage and arranged to determine the differences between successive samples, and read-out means connected to the output of the subtracting means and to the logarithmic clock for providing said differences in conjunction with the number of the sample in each case.
 5. An apparatus according to claim 4 in which the sampling stage comprises a control gate operable according to control signals from the logarithmic clock, a counter, a linear clock producing a continuous train of pulses of constant frequency, said pulses being fed to both the counter and the subtracting means via said control gate, and a comparator for comparing the signal stored in the counter with the inputs signal, said comparator being arranged to apply an inhibit signal to said control gate when the analogue of the signal stored in the counter is greater than or equal to the inputs signal.
 6. An apparatus according to claim 5 in which the logarithmic clock provides an inhibit signal of fixed duration to the control gate at each of the logarithmically separated times.
 7. An apparatus according to claim 5 in which the subtracting means is a second counter controlled by the logarithmic clock, the pulses supplied from the linear clock via the control gate being fed to said second counter during each of the sampling intervals; the number of the pulses accumulating during each interval being fed to the read-out means and the counter reset on a signal from the logarithmic clock applied to the counter during the period in which the inhibit signal is being applied to the control gate by the logarithmic clock.
 8. An apparatus according to claim 7 in which the read-out means comprises a digital to analogue converter and an oscilloscope, the signal from the second counter being applied to one of the deflection axes of the oscilloscope while a signal which increases by an equal increment for each interval is applied from the logarIthmic clock to the second deflection axis of the oscilloscope.
 9. An apparatus according to claim 8 in which the oscilloscope is adapted to retain on its screen the image of each set of signals for the duration of the determination.
 10. An apparatus according to claim 1 in which the step generator is arranged to repeatedly apply a voltage pulse to the capacitor containing the specimen, each pulse having substantially the same amplitude as subsequent pulses.
 11. Apparatus according to claim 10 in which the pulse generator is arranged to apply to the capacitor a train of rectangular pulses, each pulse having the same predetermined duration and said pulse train having substantially equal increments of time between successive pulses.
 12. An apparatus according to claim 10 in which the sampler comprises: a logarithmic clock, a switch operated on a signal from said logarithmic clock, a first storage capacitor connected to the output of the detector via said switch, whereby said storage capacitor is charged substantially to the output voltage of the detector when said switch is closed, a second storage capacitor arranged to be dischargeable linearly at a controlled rate, means for charging said second storage capacitor with a voltage dependent on the voltage of said first storage capacitor, and a timed ramp analogue to digital converter operative according to the time taken to discharge said second capacitor.
 13. An apparatus according to claim 10 having processing means for obtaining at least one of the frequency dependent dielectric parameters permittivity( epsilon ''( omega )) and loss factor( epsilon ''''( omega )), said processing means comprising: a subtractor which determines the difference between successive signals representing the sample read into the data store, a multiplier in which said difference between successive signals is multiplied by one or more of the required coefficients x(n) or y(n) for the determination of permittivity or loss factor respectively, and the products thus obtained are summed in respect of each sample obtained, display means connected to the multiplier for displaying the summations produced in conjunction with the time interval used during sampling, and a control which controls the functioning of the processing means; wherein the coefficients are defined according to the equations
 14. An apparatus according to claim 13 in which the display means is a plotting mechanism in which the summation from the processing means is plotted against the time interval number.
 15. A method form measuring the charge-time response of an insulating material to a voltage step applied across it, which method comprises applying to a capacitor containing a specimen of the material as dielectric, a voltage step or repeated voltage steps in the form of a pulse train, and sampling the charge on the specimen after a time interval from the application of the voltage step, and repeating the sampling at subsequent time intervals from the same or subsequent voltage step application, said time intervals increasing logarithmically with successive samples.
 16. A method according to claim 15 which comprises an equal voltage step of opposite polarity to a reference capacitor in which the dielectric has substantially no dielectric loss, and the quantity of charge measured is the sum of the charges which flow into the sample and into the reference capacitor.
 17. A method of determining the dielectric loss of a material over a range of frequencies which comprises applying a unidirectional voltage step across a sample of the material whereby the potential difference across the sample is changed from a first value to a second value, measuring the quantity of charge which flows into the sample at said second value in the intervals between logarithmically separated times, and computing the loss factor ( epsilon '''') with frequency ( omega ) according to the equation where q(t) is the quantity of charge flowing through the sample at a time t and A is a constant.
 18. A method according to claim 17 which includes transforming the charge into a voltage proportional to the charge, and measuring the change in the voltage which occurs during the intervals between logarithmically separated times, the value obtained being used in place of the charge in computing the loss factor.
 19. A method according to claim 17 which comprises plotting a signal proportional to the charge which flows for each interval along a first axis against the logarithm of the time of commencement of the interval along a second axis whereby the first axis is proportional to the dielectric loss and the second axis is proportional to the logarithm of the frequency.
 20. A method according to claim 19 which includes applying the signals proportional to the charges to one of the deflection axes of an oscilloscope and applying the logarithm of the commencement of the intervals to the other deflection axis of the oscilloscope to display the sequence of signals thereon.
 21. A method of determining dielectric parameters over a range of frequencies, comprising repeatedly applying a voltage step in the form of a pulse train to a capacitor containing a specimen of the material as dielectric, measuring the quantity of charge flowing into the sample after a time interval from the application of each voltage step such that successive time intervals are increased logarithmically, and computing one or both of the dielectric parameters permittivity ( epsilon ''( omega )) and loss factor( epsilon ''''( omega )) respectively with frequency according to the equations
 22. A method according to claim 21 which comprises determining the difference between the values of successive samples, multiplying each difference signal by the required coefficient, x(n) or y(n) respectively, and when more than one value for the coefficient is required for each sample, multiplying the value of the difference signal by each of the coefficients and summing the results, and displaying the result of the multiplication or the sums of the multiplications as the case may be for each sample, in conjunction with the time interval number.
 23. A method according to claim 22 which comprises displaying the values of permittivity and loss factor by plotting the computed summations against the time interval number to give a graph of epsilon '' or epsilon '''' respectively against the logarithm of the frequency. 